NASA officials aren't sure what triggered a leak of water into a spacewalking astronaut's suit, causing mission controllers to abort a planned 6.5-hour spacewalk after only one hour and 32 minutes Tuesday (July 16).

European Space Agency astronaut Luca Parmitano reported "a lot of water" inside his helmet during his excursion outside the International Space Station (ISS) Tuesday. Because of the potentially dangerous situation, mission controllers on the ground decided to abort the spacewalk.

NASA astronaut Chris Cassidy and Parmitano returned safely to the U.S. Quest airlock, and engineers began trying to figure out what caused the mysterious and unprecedented leak. [See photos from the aborted spacewalk]

"Clearly we have a problem at this point that we don't quite understand," Kenneth Todd, the ISS mission management team chair, told reporters Tuesday.

NASA officials have already made some progress in narrowing down the source of the leak. Initially, Parmitano and Cassidy thought that the astronaut's drinking water may have been leaking into the suit. However, the spacewalk team says that the cooling system in Parmitano's suit is a more likely candidate.

The spacewalk by Parmitano and Cassidy began at 7:57 a.m. EDT (1157 GMT) and was scheduled to last 6.5 hours. However, it ended at 9:29 a.m. EDT (1329 GMT) — just 92 minutes in — because of the water in Parmitano's helmet.

"There is some in my eyes, and some in my nose," Parmitano said during the spacewalk. "It's a lot of water."

Officials have now determined that a total of 34 to 51 ounces (1 to 1.5 liters) of water leaked into Parmitano's spacesuit. Most found its way into his helmet, but other parts of his suit were affected as well, Eversley said.

Parmitano and Cassidy were scheduled to continue to prepare the outside of the orbiting laboratory for the arrival of a new Russian multipurpose laboratory module expected to arrive later this year, as well as work on a variety of other maintenance tasks.

NASA officials have said that the scheduled spacewalk activities are not urgent and can be completed during a later spacewalk, but nothing has been scheduled as of yet.

Today's spacewalk was the second in eight days for Parmitano and Cassidy and the 171st excursion supporting building and maintenance of the International Space Station. It marked Cassidy's sixth spacewalk and Parmitano's second. Cassidy has clocked 31 hours and 13 minutes of spacewalking time while Parmitano holds seven hours and 39 minutes.

Both spacewalkers have re-entered the International Space Station following the cancellation of a spacewalk on July 16, 2013.Credit: NASA TV

NASA's Karen Nyberg and Russian cosmonauts Fyodor Yurchikhin, Pavel Vinogradov and Alexander Misurkin share space with Parmitano and Cassidy aboard the orbiting lab. Together, the six spaceflyers make up the crew of Expedition 36 .

The $100 billion International Space Station is the product of five space agencies representing 15 different countries. Construction of the laboratory began in 1998, and rotating crews of astronauts have continually staffed the orbiting laboratory since 2000.

Miriam Kramer

Miriam Kramer joined Space.com as a staff writer in December 2012. Since then, she has floated in weightlessness on a zero-gravity flight, felt the pull of 4-Gs in a trainer aircraft and watched rockets soar into space from Florida and Virginia. She also serves as Space.com's lead space entertainment reporter, and enjoys all aspects of space news, astronomy and commercial spaceflight.
Miriam has also presented space stories during live interviews with Fox News and other TV and radio outlets. She originally hails from Knoxville, Tennessee where she and her family would take trips to dark spots on the outskirts of town to watch meteor showers every year. She loves to travel and one day hopes to see the northern lights in person. You can follow Miriam on Twitter and Google+.